1. The Field of the Invention
This invention relates to a liquid crystal display, and more particularly to a back light unit in the liquid crystal display that minimizes a reflection of the pattern on a wall surface of a light-guide plate and bright lines from a light input.
2. Description of the Related Arts
Generally, a liquid crystal display (LCD) controls an amount of light transmitted from a back light unit. The transmission is controlled by means of a liquid crystal panel including a number of liquid crystal cells arranged in a matrix and a number of control switches for switching video signals to be applied to the liquid crystal cells, thereby displaying a desired picture on a screen. Conventional back light units will be described with reference to FIG. 1 and FIG. 2.
Referring to FIG. 1, the first conventional back light unit includes a light-guide plate 4 for guiding light passing through a light input 20; a reflective plate 2 disposed under the light-guide plate 4 for reflecting light escaping from a lower and side surfaces of the light-guide 4 in an upper direction toward an upper surface of the light guide 4; a first diffusion sheet 6 for diffusing light passing through the light-guide plate 4; first and second prism sheets 8 and 10 for controlling the direction of light passing through the first diffusion sheet 6; and a second diffusion sheet 12 for diffusing light passing through the prism sheets 8 and 10.
The light input 20 includes a lamp 22 for generating light, and a lamp housing 24 for housing the lamp 22. The lamp housing also reflects the tight into the light-guide plate 4. A printed pattern is provided on the lower surface of the light-guide plate 4. This printed pattern does not allow the light-guide plate 4 to exhibit total internal reflection, which would allow light to be uniformly distributed out of the upper surface of the light-guide plate 4.
As mentioned above, the light escaping from the lower surface and the side surface of the light-guide plate 4 are redirected by the reflective plate 2. The light passing through the light-guide plate 4 are diffused into an entire surface area of a liquid crystal panel (not shown) by the first diffusion sheet 6.
The light entering the liquid crystal panel at right angles has a large light efficiency. Thus, it is preferred that the light enter the liquid crystal panel perpendicular to the surface of the liquid crystal panel. Towards this end, two forward prism sheets are disposed to make the angle of the light exiting from the light-guide plate 4 perpendicular to the liquid crystal panel. Referring to FIG. 1, the light passing through the first and second prism sheets 8 and 10 is incident to the liquid crystal panel via the second diffusion sheet 12.
The first conventional back light unit cannot obtain a desired visual angle profile until the two prism sheets are included. The extra prism and the diffusion sheets absorb more light and thus cause an increased loss of light being transmitted to the liquid crystal panel. Also, the manufacturing cost rises.
A suggested structure to solve the above-mentioned problems is shown in FIG. 2. The second conventional back light unit includes a light-guide plate 4′ for guiding light passing through a light input 20; a reflective plate 2 disposed under the light-guide plate 4′ for reflecting light escaping from a lower and side surfaces of the light-guide 4′ in an upper direction toward an upper surface of the light guide 4′; a backward prism sheet 14 for controlling the direction of light passing through the light-guide plate 4′; and a diffusion sheet 12 for diffusing light passing through the prism sheet 14.
The light input 20 and the reflective plate 2 have the same function and operation as those in FIG. 1.
A prism-shaped pattern is provided on the lower surface of the light-guide plate 4′. This prism-shaped pattern does not allow the light-guide plate 4′ to exhibit total internal reflection, which would allow the light to be uniformly distributed out of the upper surface of the light-guide plate 4′.
In this case, it is desirable that, since the angle of the light outputted from the light-guide plate 4′ is more than about 65°, vertical angles of the prism sheet 14 should be between 63° to 70°. Thus, the light passing through the prism sheet 14 make right angles with respect to the surface of the liquid crystal panel. The light passing through the prism sheets 14 are diffused into the entire surface area of the liquid crystal panel by the diffusion sheet 12.
In the second conventional back light unit, the wall surface of the light-guide plate 4′ are reflected and the bright lines of the light input 20 are seen due to the backward prism sheet 14. To solve the problems of the conventional art, a new scheme is needed to reduce the manufacturing cost as well as minimize the wall surface reflection and the bright lines of the light input.